1. Field of the Invention
This invention relates to an electro-luminescence display (ELD), and more particularly to an apparatus and method for driving, and a method of fabricating an electro-luminescence display panel.
2. Description of the Related Art
In general, flat panel display devices have a reduced weight and size. Because of the reduction in weight and size, flat panel display devices eliminate some disadvantages associated with cathode ray tubes (CRT). Such flat panel display devices include liquid crystal displays (LCD), field emission displays (FED), plasma display panels (PDP) and electro-luminescence (EL) displays.
An EL display is a self-luminous device capable of emitting light by re-combination of electrons with holes in a phosphorous material. The EL display has some of the same advantages as a CRT in that it has a faster response than a passive-type light-emitting device like LCD, which requires a separate light source. EL displays are classified into current driving systems and voltage driving systems.
FIG. 1 is a section view showing a structure of an organic light-emitting cell in a general electro-luminescence display panel in accordance with a related art. Referring to FIG. 1, the organic EL device of the EL display includes an electron injection layer 4, an electron carrier layer 6, a light-emitting layer 8, a hole carrier layer 10 and a hole injection layer 12. The layers are sequentially disposed between a cathode 2 and an anode 14.
When a voltage is applied between a transparent electrode (the anode 14), and a metal electrode (the cathode 2), electrons produced from the cathode 2 are moved through the electron injection layer 4 and the electron carrier layer 6 into the light-emitting layer 8. Concurrently, holes produced in the anode 14 are moved through the hole injection layer 12 and the hole carrier layer 10 to the light-emitting layer 10. Thus, electrons and holes fed from the electron carrier layer 6 and the hole carrier layer 10, respectively, recombine at the light-emitting layer and generate light. The generated light is emitted outside of the device through the transparent electrode (the anode 14) to thereby display a picture.
FIG. 2 is a block diagram showing a configuration of a driving apparatus for the general electro-luminescence display panel in accordance with the related art. Referring to FIG. 2, the related art active matrix type EL display includes an EL display panel 16 having pixel (hereinafter “PE”) cells 22 arranged at each crossing of one of the gate electrode lines GL and one of the data electrode lines DL. The EL display panel also includes a gate driver 18 for driving the gate electrode lines GL. The EL display panel further includes a data driver 20 for driving the data electrode lines DL. The EL display panel further includes a timing controller 28 for controlling the data driver 20 and the gate driver 18.
FIG. 3 is an equivalent circuit diagram of each pixel cell in accordance with the related art. Referring to FIG. 3, the related art active matrix type EL display includes an external current generating circuit 32. The external current generating circuit 32 is connected to the data electrode lines DL.
The timing controller 28 generates gate control signals GCS to control a driving of the gate driver 18 for driving the gate electrode lines GL. The timing controller 28 also generates data control signals DCS to control a driving of the data driver 20 for driving the data electrode lines DL. Further, the timing controller 28 aligns externally supplied data signals and applies them to the data driver 20.
The gate driver 18 generates gate signals for sequentially enabling the gate electrode lines GL in response to the gate control signals GCS from the timing controller 28. The generated gate signals include a start pulse and a clock signal. The gate driver 18 sequentially applies the gate signals to the gate electrode lines GL.
The data driver 20 applies data signals from the timing controller 28, through the data electrode lines DL, to the pixel cells 22 in response to the control signals from the timing controller 28. The data driver 20 applies data signals for each horizontal line to the data electrode lines DL every horizontal period when the gate driver 18 drives each of the gate electrode lines GL.
Each pixel cell 22 is selected when a gate signal is applied to a cathode (the gate electrode line GL). The selected pixel cell generates light in accordance with a pixel signal, which is a current signal. The pixel signal is supplied to an anode, data electrode line DL. Each pixel cell 22 can be equivalently expressed as a diode connected between the data electrode line DL and the gate electrode line GL. Such a pixel cell 22 is driven by a gate signal, which is enabled on the gate electrode line GL. Thus, the pixel cell generates light in accordance with a magnitude of the data signal on data electrode line DL.
Each pixel cell 22 includes a supply voltage line VDD, a light-emitting cell OLED and a light-emitting cell driving circuit 30. Each pixel cell also includes a light-emitting cell driving circuit 30. The light-emitting cell OLED is connected between the supply voltage line VDD and a ground voltage source GND. The light-emitting cell driving circuit 30 drives the light-emitting cell OLED in response to a driving signal from each of the data electrode lines DL and the gate electrode lines GL.
As shown in FIG. 3, the light-emitting cell driving circuit 30 includes a driving thin film transistor (TFT) T1 connected between the supply voltage line VDD and the light-emitting cell OELD. The light-emitting cell driving circuit 30 also includes a first switching TFT T3 connected to the gate electrode line GL and the data electrode line DL. The light-emitting cell driving circuit 30 further includes a second switching TFT T4 connected to the first switching TFT T3 and the gate electrode line GL. The light-emitting cell driving circuit 30 further includes a converter TFT T2. The light-emitting cell driving circuit 30 further includes a storage capacitor Cst connected between a gate terminal of each of the driving TFT T1 and the converter TFT T2 and the supply voltage line VDD. Herein, the TFT is a p-type electron metal-oxide semiconductor field effect transistor (MOSFET).
The converter TFT T2 is connected between a node positioned between the first switching TFT T3 and the second switching TFT T4, and the supply voltage line VDD. The converter TFT T2 forms a current mirror circuit with respect to the driving TFT T1. Thereby, the converter TFT T2 converts a current into a voltage.
A gate terminal of the driving TFT T1 is connected to the gate terminal of the converter TFT T2. A source terminal of the driving TFT T1 is connected to the supply voltage line VDD. A drain terminal of the driving TFT T1 is connected to the light-emitting cell OLED.
A source terminal of the converter TFT T2 is connected to the supply voltage line VDD. A drain terminal of the converter TFT T2 is connected to a drain terminal of the first switching TFT T3 and a source terminal of the second switching TFT T4.
A source terminal of the first switching TFT T3 is connected to the data electrode line DL. A drain terminal of the first switching TFT T3 is connected to a source terminal of the second switching TFT T4, which is also connected to the drain terminal of converter TFT T2, as set forth above. A drain terminal of the second switching TFT T4 is connected to the gate terminal of driving TFT T1, the gate terminal of converter TFT T2 and the storage capacitor Cst. A gate terminal of each of the first switching TFT T3 and the second switching TFT T4 is connected to the gate electrode line GL.
The converter TFT T2 and the driving TFT T1 are presumed to have the same characteristics and are disposed adjacent to each other to form a current mirror circuit. Thus, a current amount flowing in the converter TFT T2 is equal to a current amount flowing in the driving TFT T2 when the converter TFT T2 has the same width to length dimension ratio as the driving TFT T2.
A driving of such a light-emitting cell driving circuit 30 is described as follows. First, if a gate ON signal is applied to the gate electrode line GL, then the first switching TFT T3 and the second switching TFT T4 are turned on. Subsequently, a data signal from the data electrode line DL is supplied through the first switching TFT T3 and the second switching TFT T4. The data signal turns on each of the driving TFT T1 and the converter TFT T2. Thus, the driving TFT T1 controls a current between the source terminal and the drain terminal thereof. The current is fed from the supply voltage line VDD in response to a data signal applied to the gate terminal of driving TFT T1. The driving TFT T1 applies the controlled current to the light-emitting cell OLED, thereby causing the light-emitting cell OLED to radiate with a brightness corresponding to the data signal.
Concurrently, the converter TFT T2 is connected, through the first switching TFT T3 and the data electrode line DL, to the external current generating circuit 32. Thus, a current id from the supply voltage line VDD is sunk, through the converter TFT T2 and the first switching TFT T3, into the external current generating circuit 32. When the current id from the supply voltage line VDD is being sunk into the external current generating circuit 32, a current flowing in the driving TFT T1 is equal to a current flowing in the converter TFT T2. This is because the driving TFT T1 and the converter TFT T2 form a current mirror circuit.
The storage capacitor Cst stores a voltage from the supply voltage line VDD depending upon an amount of the current id from the supply voltage line VDD sunk into the external current generating circuit 32. In other words, the storage capacitor stores a voltage between the gate terminal and the source terminal of the converter TFT T2 when the current id from the supply voltage line VDD is being sunk into the external current generating circuit 32.
On the other hand, if a gate OFF signal is applied to the gate electrode line GL, then the first switching TFT T3 and the second switching TFT T4 are turned off. Subsequently, the storage capacitor Cst drives the driving TFT T1 due to the stored voltage to thereby apply a current to the light-emitting cell OLED.
Such a related art active matrix type EL display can eliminate a stripe phenomenon generated between the adjacent pixel cells 22 due to a non-uniformity of the TFT's caused by a characteristic difference between poly silicon films configuring the TFT's by driving the EL display panel using the current-driving data driver. However, the related art active matrix type EL display has several drawbacks. For example, the related art active matrix type EL display includes four TFT's for driving the light-emitting cell OLED of each pixel cell 22. It also has a low aperture ratio when light is emitted from the light-emitting cell OLED through the anode, which is the transparent electrode.